Bimolecular orientation is a process whereby, applying a mechanical deformation to a pipe or blank previously extruded in suitable conditions of temperature, pressure, deformation speed and deformation radius principally, a substantial modification of its mechanical properties occurs, principally the sigma (or stress) of the material, resistance to impact, creep improvement, crack propagation resistance, improvement of Young's module, etc.
With said molecular orientation process, an ultraresistant pipe is produced, with less raw material and with identical or superior features, thanks to the better resistance of the material.
To reinforce the pipe tangentially, which is in the direction where it is intended to reinforce the material to withstand pressure, the previously extruded pipe should be expanded radially, thereby increasing its diameter considerably.
There are various systems for the manufacturing of tubular profiles, which can be grouped in two large categories: Continuous or line systems and discontinuous or “in-batch” systems
Bearing in mind that the apparatus and method of the invention is included in the category of the second system stated, allusion will mainly be made to discontinuous or “in-batch” systems which consist of processes which produce molecular orientation “element by element” based on an expansion of the tubular blank within a mould which provides the definitive form of the tubular profile.
There are numerous patents and documents which disclose variants of this system or method, it being possible to cite:                Method based on patent of invention WO98/13190. The hot pipe is secured at the ends of the mould within a tightened sleeve subjected to internal pressure to, in a first phase, adhere the pipe against the sleeve walls, which is a temperature similar to the orientation temperature, and then, and in a second phase, remove the sleeve by axial sliding, so that the plastic pipe expands radially and adhering to the walls of the mould, which is cold, achieving the cooling of the pipe by contact with said cold mould.        
The main characteristics of said method consist of the mould being composed of two concentric bodies which slide one within the other, performing the expansion in two phases and producing the cooling by contact with the mould walls.                Method based on patent of invention U.S. Pat. No. 4,340,344 where the pipe is introduced in a symmetrical mould, it is heated to pass hot water inside and outside the plastic blank and, when orientation temperature is reached, the inner pressure increases and the blank expands on the mould.        
The pipe is cooled as in the previous case by contact with the mould, when it is expanded, i.e. externally and indirectly, with the special feature that the mould has been heated first with water used to heat the plastic pipe and then the mould is cooled to cool the plastic pipe, which determines a very poor energy efficiency of the general process.
The first method is carried out by moulds formed by systems with actuations and internal mechanisms of certain complexity, so that in its normal functioning and due to the fact that they are concentric and are displaced axially, they reach double their length, an expensive system being necessary with a great need for space. Likewise, the inner sleeve which supports the blank plastic pipe in first instance should be hot, for which reason it is necessary that it permits the recirculation of the tempering hot fluid or a system of electric resistances and their connections, further complicating the mould.
Likewise, said mould is always kept cold thanks to an external bath of cooling fluid, whilst the cooling of the already formed plastic pipe occurs indirectly through this mould and by contact, which gives an energy inefficient system, and disadvantageous as regards processing time, since as the plastic has a dilatation coefficient greater than that of metal, a contraction occurs almost immediately and, therefore, the separation between the mould and the plastic, enormously hindering the transmission of heat by contact. The expansion occurs in at least two phases, being a slow process which is subordinate to the displacement of the moving sleeve.
In the same manner, the heat efficiency is worsened in the second method, since the mould is heated and cooled each time a pipe is processed, which involves a high energy cost, determining a low efficiency as in the previous case, since again the cooling occurs by contact. In this method, the pipe is introduced cold, the mould serving as heater and as mould in itself, having numerous orifices to permit the evacuation of the fluid which has previously served to heat the pipe, thus making the execution of the mould more expensive.
Patent of invention WO 98/56567 discloses an oriented plastic pipe manufacturing process which consists of a means of securing of the tubular blank, and it has an evacuation system by micropores of a fluid which heats the tubular blank, the mould having an intermediate sleeve with purposes of heating and subsequent cooling of the pipe mould.
A process of producing tubular premouldings of a thermoplastic material suitable for shaping to form containers by rolling process is disclosed in the U.S. Pat. No. 4,530,811, wherein the material is a part of a tube is axially oriented as a result of reducing the wall thickness by applying an external pressure which causes the material to flow.
Japanese patent JP9136325 discloses a system and method to control the temperature of a mould for blow moulding, the system is based in the introduction of vacuum in chamber external to the mould which is connected to the inner chamber using aerators. The mould is further cooled down by introducing cooling water in said external chamber.
U.S. Pat. No. 6,099,285 describes a process and apparatus for moulding, in which a molten thermoplastic resin is fitting onto a moulding surface of a mould under a certain pressure and it's then cured to obtain a moulded product.